Generally you will want to use a benchmark utility like fio. When measuring the amount of IOPs keep in mind that you need to think about things such as:

Block size

If these are writes, reads, or a mixture

If the activity is sequential or random

The number of outstanding requests

These will all effect the amount of IOPs you get. You want to make sure that with these tests you consider the response time it takes to fulfill these requests in the results.

Also remember when performing these tests you want to make sure that you flush the file cache (Howto here: deleting linux cached ram) before each test and that your test file is larger than any controller/disk caches.

I'm just about to create an ext4 partition for the vm I'm going to be testing the I/O on. Mkfs.ext4 allows for many options. Is there anything special I should specify if I only care about performance and not about slack? Like block size?
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HennoFeb 12 '11 at 15:56

@Henno: That will probably be best to ask as a new question: "What ext4 options should I consider tuning for performance when making a new filesystem?" or something like that ...
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Kyle Brandt♦Feb 12 '11 at 16:13